Hard Armor, Riprap and Gabions
The more extreme the forces pushing or pulling against soil, whether it’s gravity, wind or water, the harder the armoring needs to be. When severe scour from fast-moving water flows are eating away at the sides of a channel, or a very steep slope is in danger of sliding, it’s time to turn to the hard stuff.
Hard armoring comes in many different forms. Riprap, which consists of piles of angular rocks; gabions, which are wire baskets filled with riprap rock or broken concrete; and gabion (Reno) mattresses, are among the oldest and best-known types.
Mechanically-stabilized earthen (MSE) walls are another classic example of hard amoring. Cellular containment systems, depending upon how they’re used, can also be considered hard armoring.
Some newer types utilize man-made rock, in the form of articulated concrete block (ACB).
Whether in the form of three-dimensional jacks, stepped overlays, interlocking wall arrangements, or cabled together in flexible mats, ACBs have proven to be very effective against scour and extreme erosive forces. Many of the older—and newer—methods can be further bolstered and beautified by vegetating over them.
Riprap and matrix riprap
Riprap, also known as rubble, shot rock or rock armor, may look like a pile of rocks, but it’s much more complicated than that. First of all, the rocks used in a riprap installation aren’t just any old rocks, but ones that were chosen for size and angularity. Rounded river rocks won’t work.
It’s a classic form of hard armoring, used mainly to protect shorelines, streambeds, bridge abutments and pilings against scour and ice damage. A variety of different rocks are used, including granite, limestone and broken concrete rubble.
The rocks absorb and deflect the impact of waves, and the voids between the rocks slow down the flow of water and allow filtration. Riprap rocks need to be angular, because when they’re piled atop one another, friction helps them lock in place under their own weight.
A little different twist on riprap, known as ‘matrix riprap,’ involves injecting concrete into the voids between the rocks, grouting them in place. It’s cheaper than traditional riprap, because it allows you to use smaller rocks, which are cheaper to haul, easier to handle and—because of the cementing—fewer are needed.
This technique was used successfully on a project involving the East side Detention Facility, an 1,800- acre-feet flood-control reservoir in Larimer County, Colorado. It’s considered a ‘high hazard’ dam, which means that a failure or operational error could result in the loss of human life.
“On the downstream side of the spillway, there was a very intense scour situation, involving 3:1 slopes and overtopping flow,” explained Chris Pletcher, municipal engineering manager at Ayres Associates, in their Fort Collins, Colorado office.
“We looked at using different kinds of hard armor, at traditional riprap, and even erosion-control fabrics,” said Pletcher. “Matrix riprap ended up being the most cost-effective, and provided the best performance. It’s a technique that the Europeans have been using for 30 or 40 years, with great success, but it was never really brought to the U.S. We looked at the research regarding what the Europeans were doing, and put it into a format that we could use on actual construction projects.”
The concrete was batched onsite, then pumped into the voids between the riprap that was stacked in the damaged areas of the dam. Pletcher compares this process to using a hot-glue gun. “By gluing the rocks together, they become more like caramel corn, and that provided a couple of advantages. The smaller rock size has smaller voids, and therefore it’s easier to filter the material that’s below.”
Not everyone likes matrix riprap, though. Some flexibility is lost when you cement rocks together, and filled-up voids can’t filter or slow the water flow. Un-grouted standard riprap can adjust to ground settlement and movement from seismic activity much better. However, in this case, the matrix technique stopped the overtopping and scour.
Gabion basket walls
Pipelines are supposed to be buried and not seen. But when an area near Grenada, Mississippi, suffered extreme soil erosion due to large storm and flooding events, portions of the large-diameter gas pipeline going through it were exposed. In addition, an adjacent steep slope had become very unstable, and the banks of a stream had eroded. Poor soils and lack of vegetation also contributed to the problem.
The project team at Trestles, LLC, a Nashville, Tennessee engineering firm, was tasked with coming up with a long-term solution: a terraced wall design that relied heavily on welded-wire gabion baskets to firm up the slope, and divert stormwater and subsurface groundwater away from the critical area.
“The pipeline comes down the hillside at a steep angle, and hits a bend at the bottom of the hill, where it crosses a creek,” explained project manager Justin Pope, P.E. “The creek had moved into the hillside, creating a near-vertical face on it, and exposing the pipe.”
This was not an easy job; in fact, Pope said that it was the most complicated, difficult erosion-control job he’s done so far. “There were three tiers. I think the initial wall was 15- to 18-feet tall, with a three-to-one slope back into the bank.”
“Then we had to build another six-foot wall, with a three-to-one slope, and then another wall to get us back to grade. Altogether, it was about 40 feet of fall from the top, terraced down to the toe of the bank, and sloped at a 45-degree angle.”
Local rock couldn’t be used to fill the baskets, as it was mostly sandstone, which eventually dissolves in water. A quantity of limestone, in the form of standard four- to sixinch diameter gabion rocks, had to be brought in by rail car.
It’s been said that only Superman can change the course of mighty rivers, but on this project, it was ordinary contractors who altered the grade of the creek. A 60- to 80-foot section of it had to be raised so that the pipe would be covered.
To bring the grade up, a BMP known as an articulating block mat was used—essentially, a big geotextile bag with an interior reinforced by stainless steel cables and pumped full of cement. A crew rolls it out, and then it’s a sequence of zipping and stitching to get it into position. The mat must be filled in place, because once the concrete is put in, it becomes much too heavy to be moved.
“We placed the mat in the creek, and pumped it full of concrete, making a new creek lining on one end of it,” recalled Pope. “We pitched it back toward the bank, and that’s where we built our wall. Ultimately, we were just putting the creek back to where it was when the pipe was originally laid in there.”
“We laid the mat in, filled it, then toed it in on the uphill side of the bank. On the opposite side of the bank, we stacked a gabion wall, and stepped it back to reduce the amount of backfill required.”
After that, the area was hydroseeded. Subsequent rain events proved the durability of the installation. “Before, the entire slope was sloughing off, because there was no vegetation and the sandy soil was constantly eroding,” said Pope. “Now it’s hard armored, but you’d never know, because all you see is a gentle slope with grass on it.”
MSE walls stabilized with infilled wire mesh
The Harold Court East Regional Service Center in Austin, Texas, is a multipurpose city-owned facility shared by several government departments. It’s used for office space, material and equipment storage, and vehicle maintenance.
In 2012, the slopes on the western and southern boundaries were found to be unstable, resulting in periodic slides. Soil was migrating into Fort Branch Creek, a watershed to the Colorado River. The slides were also a threat to the safety of workers at a nearby materials storage yard.
In addition, the shifting embankment had damaged an existing 48- inch storm drain outfall at the bottom of the western slope. Any further lateral movement could damage an existing wastewater main in the area. A permanent form of slope stabilization was needed.
This project was no walk in the park. Many unforeseen curveballs lay ahead. For starters, there was the fact that, for 50 years, the site had been a dumping ground for the city and its residents.
“It wasn’t clear how much hazardous material was going to be found and have to be disposed of before anything could be built,” said Marco Invernizzi, P.E., engineering consultant and owner of MAIN Geo- Construction Sources and Alpi Engineering, LLC. “A geotechnical investigation had revealed all sorts of stuff in that pile, and there was a big misunderstanding as to how any hazardous material found in there should be treated.”
It’s very expensive to dispose of hazardous wastes. Since it was unclear exactly how much might be found in the massive pile, the first contractor withdrew his bid. (In the end, no toxic or dangerous materials were actually found.) Austin Filter Systems, Inc., was ultimately awarded the job.
The project was supposed to be completed in 330 days. However, a series of unforeseen events stretched that to 800 days—more than two years.
A slide occurred during the initial excavation, which threatened to topple a nearby high-voltage tower. “Before they could go any further, they had to stabilize the area around that tower,” said Invernizzi. “That was the reason for the long delay.”
The cause of this slide—and all the others—was discovered: seepage from an unexpectedly high water table.
“We sat down with the owner to give him our recommendations,” said Craig Bond, operations manager for Austin Filter Systems. “One of them was to build an underdrain system, which is what we ended up doing. It was the most economically feasible way to channel the water away from the site.”
Another delay was caused when a very large pipe, part of an old, undocumented storm sewer, was discovered. This had to be dug out and removed.
The original plan was to build a series of nine-foot-tall gravity-retaining walls along the steep slope, with ten-foot terraces in between. But all of the challenging onsite conditions lead the designer to modify that plan, and instead specified several nine-foot-tall MSE retaining walls separated by vegetative terraces.
The terraces would be reinforced by pre-assembled units of double-twisted wire mesh. The facing section of each unit is formed by connecting a back panel and diaphragms to the main fascia (facing) unit, thus creating rectangular cells used for stone confinement.
The geogrid reinforcement, fascia and lid are all one continuous panel of mesh.
Following assembly onsite, the facing units were filled with suitable gabion stone fill. Structural backfill was then placed upon the soil reinforcement geogrids and compacted.
Subsequent mesh layers were placed on the completed layers beneath. And the slides stopped.
Cellular containment system
To enhance the enjoyment of both its residents and visitors, the city of Ocean Springs, Mississippi, on the Gulf Coast, had recently constructed a beachside sidewalk. But rainstorms and high tides were washing away the sand next to it, carving a dangerous and un-scenic 12- to 18- inch drop-off from the sidewalk to the beach.
This was hazardous to anyone walking, riding a bike, or driving an off-road vehicle onto the beach from the sidewalk. Previous attempts to address this problem had failed. First, a retaining wall had been erected along the sidewalk, with small openings in it, to release rain and tidewater. But whenever water flooded through those openings, the sand washed out.
“If they’d kept on extending that wall, they’d have just kept pushing that erosion further down the beach,” said supply and installation contractor Terry Jones, vice president of Geo-Products, Inc., in Jackson, Mississippi.
Since the wall wasn’t working, the city next tried lining the sidewalk’s edge with snakelike, self-weighted sediment-filtering tubes filled with shredded, recycled tires. These, too, failed to solve the problem.
The project engineers decided that a drainage layer, consisting of an eight-inch deep cellular confinement system made of high-density polyethylene strips formed into honeycomb-shaped cells, would be the best solution to keep the sand from washing out during rain events and high tides.
A ten-foot test section was built and left in place for four months. It performed so well that the city gave the go-ahead to install the product along the length of the entire sidewalk, some 7,500 square feet in all.
First, a large drainage ditch was excavated adjacent to the concrete sidewalk. A nonwoven geotextile fabric was placed in the bottom of the ditch and connected to the sidewalk. A drainpipe was installed on top of the fabric, and the ditch was filled in with angular rock.
The cellular confinement system was installed on top of the ditch, extending out onto the beach. It, too, was then infilled with a combination of angular rock (over the drainage area), and sand.
Would it work? The team didn’t have to wait long to find out. Just two days after they finished, the beach was hit by a torrential rainstorm that dropped four inches of water in just seven hours. Though the beachfront looked like a river, the cellular confinement system held the sand in place and also greatly improved drainage at the edge of the sidewalk. Mission accomplished.When hard armoring is needed, we have a lot more choices today than we had 30 or 40 years ago. However, it’s also a safe bet that riprap and gabions, with us for centuries, are going to continue to be important bulwarks against the forces of nature for some time to come.